Recent years have witnessed ever-increasing interest in radio positioning and navigation systems, with interest spanning a very broad range of government and commercial applications. Probably the best known, and most applied system to date is the U.S. Government Global Positioning System (GPS), which relies on many simultaneous, and globally distributed satellite broadcasts. All satellite transmissions rely on highly stable, and mutually synchronized clocks, and apply well-established spread-spectrum signaling techniques. In the absence of blockage (e.g., from buildings in urban areas) or multipath, suitable equipped radio receivers can provide highly accurate position (e.g., 100 meter to even centimeter accuracy). GPS performance, however, degrades rapidly when blockage and/or multipath is present, thereby dramatically reducing its effectiveness for a broad range of personal services in urban and suburban areas--including traveler services and FCC-mandated, wireless 911.
In recognition of the importance of enabling positioning in urban/suburban regions, considerable effort has been expended in recent years in defining and developing terrestrial-based approaches that can overcome or mitigate propagation phenomena in such difficult regions. These include: passive or active positioning via the evolving digital cellular and PCS networks; positioning via existing terrestrial AM radio transmission; positioning via existing FM radio transmission; positioning via existing TV transmissions; hybrids of GPS and terrestrial signaling. Each of these approaches offers benefits but also limitations. For example, the IS-95 spread-spectrum signal is ideally suited for ranging and positioning, but the power control inherent in the system limits the ability to simultaneously receive from multiple, geographically distributed cell-sites/base stations; multipath degradation is also a factor if very accurate positioning is desired. On the other hand, FM and TV transmissions offer very high power, but multipath, range resolution and/or geometry may limit positioning accuracy. Tracking of AM carriers offer extremely attractive resolution, and the long wavelength of AM makes it fairly immune to multipath, but AM is very sensitive to burst noise and nighttime propagation phenomena. In addition, several of these approaches require additional terrestrial infrastructure and interfaces to enable calibration of inherent error sources (e.g., oscillator clock biases).
What is clearly needed is a system approach and implementation that simultaneously draws upon the strengths of several of the above, and does so in a manner that simultaneously mitigates inherent weaknesses of any one of the approaches. In other words, two or more approaches are applied in a complementary fashion, and leverages inherent capabilities not exploited to date. Furthermore, the system approach must minimize infrastructure/interface augmentation.
The GPS system is an example of prior art in regards to positioning systems. It is a spread spectrum system where each satellite transmits a unique Pseudo Noise (PN) sequence. A GPS receiver correlates to these different PN sequences generating an impulse signal that mark the arrival time from each satellite signal. The time differences between these signals are then used to calculate a position. To maximize position accuracy, these PN Sequences are selected for their good auto-correlation properties (i.e., close to impulse). It can be shown that an OFDM symbol also has an excellent auto-correlation properties, and can be used in a similar but unique way.
It is easiest to understand the OFDM's excellent auto-correlation property by examining the signal in the frequency domain. An OFDM symbol is typically constructed by placing a symbol in each carrier (i.e., frequency bins), performing an Inverse Fast Fourier Transforms (IFFT), and transmitting the time domain signal. Assuming PSK modulation, the OFDM symbols have a constant magnitude across its bandwidth and "information" on the phases on each carrier. If the phase "information" is removed, then the resulting signal has a constant magnitude and phase. This resulting signal is the frequency response of a discrete impulse. Note that removal of phase information can be performed by bin-wise multiple across the OFDM symbol with a complex conjugate of itself. This operation is the equivalent to a circular auto-correlation in the time domain.
It should be noted that most OFDM systems have a guard interval after (and/or before) each OFDM symbol and that this guard interval contains a repeated portion (i.e., circular shift) of the OFDM symbols. This makes the system more tolerant to intersymbol caused by timing offset or multipath. It also is beneficial to positioning techniques using the circular correlation operation.